1. Field of the Invention
This invention relates to an image reproducing apparatus, such as is used in an electrophotographic copying machine.
2. Description of the Prior Art
In an image reproducing apparatus having an intermediate transfer member, a toner image on a toner image retaining member, formed by developing an electrostatic latent image with toner, is transferred to said intermediate transfer member, generally an endless belt, and is then again transferred and fixed to a transfer sheet material, such as transfer paper, forming a picture image thereon. When such an image reproducing apparatus is used, a final picture image of a high quality can be obtained. This apparatus permits use of a so-called retention system, in which an electrostatic latent image once formed can be utilized several times by subjecting said image to developing and transferring operations repeatedly to obtain a plurality of identical final picture images, allowing the reproduction of picture images to be easily carried out at an increased speed. In addition, even when a one-component conductive toner is used as a developer, a toner image can be transferred to an ordinary paper. Accordingly, a developing system can be simplified, and the quality of a picture image can be improved.
In order to transfer a toner image formed on an intermediate transfer member to a transfer sheet material, or transfer such a toner image to a transfer sheet material and fix the same thereto simultaneously, it is necessary in general that the toner image be heated before it is transferred to the transfer sheet material. In the meantime, it is also necessary to eliminate adverse effects of heat upon the parts and elements of the image reproducing apparatus. Therefore, it is desirable for the temperature of a toner image-retaining intermediate transfer member to be maintained at as low a level as possible, yet at the same time for it to be high enough to heat the toner sufficiently. If an image-retaining member is overheated, these adverse effects include a decrease in the electric potential of an electrostatic image, the fusion of toner, and difficulty in cleaning the toner image retaining member.
In an attempt to eliminate these inconveniences, various methods have been proposed, which include a non-contact heating method, in which the radiation heat is applied to the surface of an intermediate transfer member, and a method in which a heating element consisting of a pressure roller for pressing a transfer sheet material against the surface of an intermediate transfer member is used to heat the toner via the transfer sheet material. In addition to these methods, a method has also been developed in which the contact surface area between the transfer sheet material and the intermediate transfer member is increased to improve the transfer efficiency.
However, in order to heat the toner on the intermediate transfer members employed in the above-mentioned methods to a desired level, a large heat source is required. To use a large heat source in these methods would cause increased energy consumption and various troubles due to the large amounts of heat. Since a large heat source cannot be used in these methods, the speed of the intermediate transfer members in use, and thus the image reproduction speed, is limited. Consequently it can be said that the intermediate transfer members are utilized meaninglessly in the above-mentioned methods. Moreover, the condition of an image transferred to a transfer sheet material in these methods varies greatly with the thickness and weight of the transfer sheet materials in use.